Spam in my site's Guest Book?

Over the last Month or so I

If you’re using a third party guest book on your site then the spammers often know how to find them using Google searches. I don’t know the one you’re using but perhaps it’s always called gbook.php so they just look for sites that have gbook.php on them. The reason they target specific scripts is that they know there is some way to bypass the security in the script to misuse it in ways that the site owner hadn’t intended, e.g. to send spam. They probably write scripts to search Google and then send the spam automatically to any sites they find using the script without the need for human input.